The compound meta-chloroanaline, which is a desired herbicide intermediate, has most usually been prepared from meta-chloronitrobenzene.
One method used to form meta-chloronitrobenzene involves the chlorination of nitrobenzene. The resultant product is primarily meta-chloronitrobenzene, but in combination with significant amounts of both its para- and ortho- isomers.
The separation of chloronitrobenzene isomers is inherently extremely difficult by the use of simple distillation, the boiling points of meta-, para- and ortho- chloronitrobenzenes being respectively 234.degree. C., 238.5.degree. C. and 245.5.degree. C. At least where the further product of relatively pure metachloroanaline is contemplated, one can not look to postpone complete isomer separation until after the reduction step, since both metachloroanaline and parachloroanaline boil at 231.degree. C. with orthochloroanaline boiling at 208.4.degree. C.
While distillation of chloronitrobenzene isomers can yield some significant separation, if a column having a large number of theoretical plates is employed, care must be taken in employing a still temperature which is too high, for example approaching 300.degree. C., since decomposition, and even explosive decomposition of the chloronitrobenzene can be expected.
Among the known chloronitrobenzene isomer separation processes are the following:
U.S. Pat. No. 3,311,666, to Dunn, relates to the separation of chloronitrobenzene isomers by crystallization and fractionation and describes the difficulties encountered in distillation and crystallization techniques.
U.S. Pat. No. 3,816,551, to Lee, relates to the use of crystallization in separating isomers of chloronitrobenzene. Specifically, the para isomer is separated employing continuous crystallization in the presence of water.
U.S. Pat. Nos. 2,795,620 and 2,795,621, to Bloom et al, discuss various known methods for the manufacture of metachloronitrobenzene and problems relating to chloronitrobenzene isomer separation. The patents are specifically directed to separating ortho- and para-chloronitrobenzenes from metachloronitrobenzene by means of a sulfonation process.
U.S. Pat. No. 2,245,945, to van Dejck et al, relates to a process for the separation of isomeric organic compounds, including mixture of ortho- and para-chloronitrobenzene, by the use of two selective immiscible solvents to cause distribution of the isomers between the solvents.
U.S. Pat. No. 3,051,650, to Pfennig, relates to separating chemical compounds using solvents and liquified sulfur dioxide.
While a number of compounds, including some of the compounds employed in the process of the invention have been individually known to form binary azeotropes with at least one chloronitrobenzene isomer, the state of the distillation art is not sufficiently advanced so that one can readily predict that because a compound forms a binary azeotrope, that the same compound will be useful in a ternary or quaternary system, or that because one compound is effective, that another apparently closely structurally related compound will be effective. Therefore, most azeotropes are chance discoveries.
The following binary systems have been reported in "Azeotropic Data" No. 6 of the Advance in Chemistry Series. American Chemical Society, Washington D. C. (1952), at the indicated page number:
m-chloronitrobenzene/dipropylene glycol, b.p. &lt;227.0.degree. C. (page 157); PA1 o-chloronitrobenzene/triethylene glycol, no azeotrope (page 157); PA1 ethylene glycol/m-chloronitrobenzene, b.p. 192.5.degree. C., 53% glycol (page 64); PA1 ethylene glycol/o-chloronitrobenzene, b.p. 193.5.degree. C., 68% glycol (page 64); PA1 ethylene glycol/p-chloronitrobenzene, b.p. 192.85.degree. C., 57.8% glycol (page 64); PA1 propylene glycol azeotropes with unrelated compounds (page 101). PA1 ethylene glycol/m-chloronitrobenzene b.p. 192.5.degree. C. (page 84); PA1 glycerol/m-chloronitrobenzene, b.p. 232.2.degree. C. (page 84); PA1 dipropylene glycol/m-chloronitrobenzene, b.p. &lt;227.0.degree. C. (page 86); PA1 dipropylene glycol/p-chloronitrobenzene, b.p. &lt;228.3.degree. C. (page 86); PA1 trimethylene glycol/p-chloronitrobenzene, b.p. &lt;234.0.degree. C. (page 86); PA1 diethylene glycol/m-chloronitrobenzene, b.p. 228.2.degree. C. (page 86); PA1 diethylene glycol/p-chloronitrobenzene, b.p. 229.5.degree. C. (page 86); PA1 diethylene glycol/o-chloronitrobenzene, b.p. 233.5.degree. C. (page 86). PA1 Y.sub.a = vaoor composition of a (mole %) PA1 Y.sub.b = vapor composition of b (mole %) PA1 X.sub.a = liquid composition of a (mole %) PA1 X.sub.b = liquid composition of b (mole %)
Maurice Lecat, Am. Soc. Sci. Bruxelles, Ser. I, Vol. 61, pages 79-98 (1947) reports the following binary azeotropes systems:
As can be seen from the above data many of the individual chloronitrobenzene isomer binary azeotropes with a particular polyol have boiling points closer to each other, than do the isomers per se. If anything, this would appear to suggest away from the use of ternary or quaternary azeotropes as disclosed below.